Citrus genome sequencing could help block citrus greening

A University of Florida-led group of international scientists has assembled the genome sequences for two citrus varieties — sweet orange and Clementine mandarin — marking a first for citrus.
The Clementine mandarin sequence is the higher quality of the two, but both are expected to help scientists unravel the secrets behind citrus diseases such as greening.

Mickie Anderson, University of Florida | Jan 18, 2011

A University of Florida-led group of international scientists has assembled the genome sequences for two citrus varieties — sweet orange and Clementine mandarin — marking a first for citrus.

The Clementine mandarin sequence is the higher quality of the two, but both are expected to help scientists unravel the secrets behind citrus diseases such as greening, a deadly threat to the state’s $9 billion citrus industry, as well as aiding those working to improve fruit flavor and quality.

Florida citrus industry officials said they were thrilled, and relieved, by the news.

“The publication of the sweet orange and tangerine genomes will accelerate the discovery of innovative solutions to a myriad of pest and disease problems that threaten citrus production,” said Dan Gunter, chief operating officer of the Citrus Research and Development Foundation Inc.

Michael W. Sparks, executive vice president and CEO of Florida Citrus Mutual, a trade organization comprised of 8,000 members, called genomics “the future of not only Florida citrus, but the entire global citrus industry.”

“It is exciting to see breakthroughs such as the release of these (genome) assemblies and I am confident the talented scientists working on this project will eventually propagate a citrus cultivar that withstands disease pressure and allows consumers worldwide to continue enjoying nutritious citrus products,” he said.

The genome sequences, the result of at least four years’ worth of study and more than $3.5 million, were announced Saturday at the International Plant and Animal Genome Conference in San Diego.

The announcement comes just weeks after a similar announcement that another international team, led by UF Institute of Food and Agricultural Sciences and Virginia Tech scientists, had published the DNA sequence for the strawberry.

The Clementine mandarin genome came from a haploid, meaning it has a single set of chromosomes. The scientists used a more detailed method of obtaining its genome sequence, which was more expensive, but provides longer strings of DNA, said UF’s Fred Gmitter, a horticultural sciences professor and Institute of Food and Agricultural Sciences faculty member who led the effort.

“For us, it means it gives you longer reads, longer pieces — so that you’re assembling a jigsaw puzzle out of a million pieces, instead of out of 25 million smaller pieces,” Gmitter said. “What’s most important is to have this high-quality, original haploid reference sequence. And we did that.”

The team that worked to obtain the gene sequence for the Clementine mandarin included scientists from the University of Florida, Italy, Brazil, France and Spain and the U.S. Department of Energy’s Joint Genome Institute (JGI). Simultaneously, work was being done to obtain the diploid sweet orange sequence by scientists from UF, JGI, Georgia Tech and 454 Life Sciences, a Roche Company.

“I’m proud that our scientists helped lead the way in this world-class research,” said Mark McLellan, IFAS’ dean for research. “We believe having these genome sequences will greatly help the state’s citrus industry, as well as citrus growers around the world.”

Since its discovery in Florida in 2005, greening has caused havoc in the citrus industry. It has wiped out some citrus crops in Asia, Africa, the Arabian Peninsula and Brazil. Greening slowly weakens and kills all types of citrus trees, while rendering fruit malformed and discolored.

But while the two new genome sequences may provide just the tool scientists need to help them solve the greening crisis, Gmitter said having them is “really much, much, bigger than that.”

Some of the possibilities, he said, include citrus trees with more beautiful fruit, better disease resistance, more phytonutrients, and tolerance for salt, bad soil or extreme temperatures.